High-pressure metal vapor discharge lamp

ABSTRACT

The invention relates to a high-pressure metal vapor discharge lamp having an ignition auxiliary means (13) externally of the discharge vessel. The external ignition auxiliary means is connected electrically to a first main electrode (5) and in the inoperative condition of the lamp extends along the wall of the discharge vessel up to at most 3 mm beyond one end of a second main electrode (6). According to the invention the external ignition element extends at the level of the first main electrode over at most 3 mm along the first main electrode from the end of the first main electrode. In this manner a regular ignition of the lamp is obtained, variation in the ignition voltage pulse required for ignition being considerably restricted.

The invention relates to a high-pressure metal vapor discharge lamphaving a discharge vessel comprising a radiation-transmitting wall and afirst and a second main electrode between which in the operativecondition of the lamp the discharge takes place, which main electrodeseach have an end which is directed toward the discharge, the dischargevessel furthermore comprising an external ignition auxiliary means whichin the operative condition of the lamp is connected electrically to thefirst main electrode and extends from the end of the first mainelectrode up to the end of the second main electrode or up to at most 3mm beyond said end, and is also present at a distance of at most 1 mmfrom the wall of the discharge vessel and at least near the second mainelectrode tightly engages said wall.

Such a lamp is known from Netherlands Patent Application 7907437. It hasbeen found that the known lamp has a good ignition when the condition issatisfied that the external ignition auxiliary means extends along thewall of the discharge vessel up to the end of the second main electrodeor up to at most 3 mm beyond said end. A second condition for goodigniting is that the external ignition auxiliary means is present at adistance of at most 1 mm from the wall of one discharge vessel andengages said wall at least near the second main electrode. It has beenfound, however, that the known lamp often ignites irregularly, that isto say that the value of the ignition voltage pulse required forigniting varies considerably for each individual lamp.

It is the object of the invention to provide a means with which suchirregular ignition is avoided.

According to the invention a lamp of the kind mentioned in the openingparagraph is characterized in that, at the area of the first mainelectrode and measured from the end of the first main electrode theexternal ignition auxiliary means axially the first main electrode overat most 3 mm.

Lamps according to the invention have been found to have a very regularignition in which the variation in the required value of the ignitionvoltage pulse is considerably restricted.

It has been found that the following stages can be distinguished whenigniting a high-pressure metal vapor discharge lamp according to theinvention:

An auxiliary discharge is formed between the second main electrode andthe wall of the discharge vessel near said main electrode and directedtowards the external ignition auxiliary means

the auxiliary discharge then expands along the wall of the dischargevessel into the proximity of the first main electrode and directedtowards the external ignition auxiliary means

the auxiliary discharge finally jumps from the wall of the dischargevessel to the first main electrode after which the auxiliary dischargeevolves into a stable arc discharge and the operative condition of thelamp is thus reached. The various stages occur under the influence ofthe so-called ignition voltage pulse which is applied between the twomain electrodes.

The metallic components of the filling of such lamps will be condensedin the inoperative condition of the lamp. A place for the saidcondensate occurring very frequently in practical cases is the part ofthe discharge vessel which, viewed from the place where in the operativecondition of the lamp the discharge takes place, lies behind a mainelectrode.

In the case of the known lamp it has been found that the auxiliarydischarge in many cases directly affects the condensate. The voltagerequired for jumping of the auxiliary discharge from the condensate tothe adjacent main electrode is particularly high in such situations. Asa result of this the ignition of the lamp is badly reproducible. It hassurprisingly been found that in lamps according to the invention theauxiliary discharge in substantially all cases directly attacks the mainelectrode with which a readily reproducible ignition of the lamp isobtained.

The external ignition auxiliary means may be constructed, for example,as a wire wound around the discharge vessel or as a strip secured to thedischarge vessel.

In an advantageous embodiment of a lamp according to the invention astrip-shaped part of the external ignition auxiliary means in theinoperative condition of the lamp near the second main electrode tightlyengages the wall of the discharge vessel at least over half thecircumference.

It has been found with this embodiment that the variation in the valueof the ignition voltage pulse required for igniting is even furtherrestricted then it would be under otherwise the same circumstances. Thismay be explained as follows. The ignition voltage required for ignitingthe lamp will be determined by the formation of an auxiliary dischargeover the track having the largest electric field strength, that ishaving the largest voltage gradient. In ideal circumstances this trackwill be formed by that point of the second main electrode which has thesmallest distance to a point of the external ignition auxiliary means.In practical circumstances, factors such as local inhomogeneities of themain electrode, the discharge vessel wall, and the auxiliary means aswell as the instantaneous composition of the gaseous filling of thedischarge vessel at the area of the main electrode also play a role inaddition to the distance. This leads to a spreading in the voltagegradient required for the formation of the auxiliary discharge betweenexternal ignition auxiliary means and second main electrode and in turnin a spreading in the ignition voltage pulse required for the ignitingwith other relevant factors being the same in otherwise the samecircumstances. The control of these factors is only partly possible.However, by giving the external ignition auxiliary means a comparativelylarge spatial extent near the second main electrode, the influence ofthe said factors on the spreading in the required ignition voltage pulsecan be restricted. It has been found that the spreading in the requiredignition voltage pulse can be very considerably restricted already witha spatial extent of the external ignition auxiliary means in the form ofa tight engagement over half the circumference against the wall of thedischarge vessel by the ignition auxiliary means.

In the inoperative condition of the lamp the external ignition auxiliarymeans in a lamp in accordance with the invention advantageously tightlyengages the wall of the discharge vessel over a length of at least halfthe distance between the ends of the main electrodes taken from thesecond electrode. Herewith it is achieved in a simple manner that duringigniting of the lamp an expansion is promoted in the direction of thefirst electrode of the auxiliary discharge formed between the secondmain electrode and the wall of the discharge vessel.

In a further embodiment of a lamp in accordance with the invention theexternal ignition auxiliary means comprises a part which is movable withrespect to the discharge vessel and which in the inoperative conditionof the lamp extends along the wall of the discharge vessel between theends of the main electrodes and in the operative condition of the lampis remote from the wall of the discharge vessel. An advantage of thisstructure is that the part of the radiation emitted by the lamp in theoperative condition of the lamp and intercepted by the external ignitionauxiliary means is restricted. In addition, migration, if any, ofconstituents of the filling of the discharge vessel through the wall ofthe vessel under the influence of electric field strength which iscaused by voltage differences between the discharge and the externalignition auxiliary means in the operative condition of the lamp iscounteracted in this manner.

In a further improved embodiment the strip-shaped part is advantageouslyrigidly connected to the discharge vessel and in the inoperativecondition of the lamp the electric connection between the strip-shapedpart and the movable part of the external ignition auxiliary means hasan ohmic resistance of at most 100Ω. It has surprisingly been found thatthe expansion of the point of attack of the auxiliary discharge alongthe wall of the discharge vessel from the strip-shaped part in thedirection of the first main electrode is not detrimentally influenced inthe case of an ohmic resistance value realised in this manner.

The invention is suitable for use both in high-pressure metal vapordischarge lamps having a discharge vessel with a ceramic wall (forexample, polycrystalline densely sintered aluminum oxide or sapphire)and in smaller lamps having a discharge vessel formed from quartz orfrom hard glass. In addition to one or more metals, for example sodiumand mercury, and one or more rare gases, for example xenon, the fillingof the discharge vessel may also comprise halides.

An embodiment of a lamp in accordance with the invention will bedescribed in greater detail with reference to a drawing.

In the drawing, reference numeral 1 denotes an outer envelope of a lampaccording to the invention having a lamp cap 2. Inside the outerenvelope is present a discharge vessel 3 shown partly broken away havinga radiation-transmitting wall 4. The discharge vessel 3 has a first mainelectrode 5 and a second main electrode 6. Each of the main electrodes5,6 has respective ends 5' and 6' which face the discharge which takesplace in the operative condition of the lamp. Main electrode 5 isconnected to a rigid current supply conductor 9 via a leadthroughconductor 7 and a current conductor 8. The rigid current supplyconductor 9 is connected at one end to a first connection contact 2a ofthe lamp cap 2 while another end in the form of a supporting brace 9'bears against the outer envelope. Main electrode 6 is connectedelectrically to a current supply conductor 12 by means of a leadthroughconductor 10 and a flexible electrically conductive wire 11, whichconductor 12 is mechanically connected directly to the leadthroughconductor 10. The rigid current supply conductor 12 is connected to asecond connection contact 2b of the lamp cap 2.

The discharge vessel 3 has an external ignition auxiliary means 13 whichcomprises a movable part 13a and a strip-shaped part 14 provided at theaxial position of the second main electrode 6. The strip-shaped part 14and therewith the movable part 13a in the inoperative condition of thelamp is connected electrically to the first main electrode 5 by means ofa conductor 15. The movable part 13a of the external ignition auxiliarymeans 13 finally is connected to the rigid current supply conductor 9 bymeans of a bimetal plate 16.

The lamp described has a discharge vessel 3 with ceramic wall 4 madefrom densely sintered aluminum oxide. The main electrodes 5 and 6 aremade from tungsten while the leadthrough members 7 and 10 are in theform of niobium sleeves. The movable part 13a of the external ignitionauxiliary means is a tungsten rod having a diameter of 0.4 mm which inthe inoperative condition of the lamp bears against the wall 4 of thedischarge vessel and against the strip-shaped part 14 under theinfluence of the bimetal plate 16. The strip-shaped part 14 of theignition auxiliary means is preferably a niobium strip having a width of3 mm which is provided against the wall of the discharge vessel over itswhole circumference, for example by means of clamping or spot welding.Other suitable materials for the strip-shaped part are inter aliamolybdenum, tantalum and titanium. The discharge vessel has a fillingcomprising 10 mg of amalgam of which 81.6% by weight of mercury and18.4% by weight of sodium. In addition to mercury and sodium thedischarge vessel comprises xenon which at approximately 300 K has apressure of 80 kPa. The lamp is suitable for operation at an alternatingvoltage source of 220 V, 50 Hz by means of a stabilisation ballast of600 mH. The power consumed by the lamp in the operative condition is 70W.

In the inoperative condition of the lamp described the movable part 13aextends from the external ignition auxiliary means 13 along thedischarge vessel and tightly engages the wall of the discharge vesselover substantially its full length. At the axial position of the firstmain electrode the movable part extends along the first main electrodeover a length of approximately 2 mm taken from the end of the first mainelectrode.

At the axial position of the second main electrode the movable part 13aextends up to 2 mm beyond the end of the second main electrode 13a andin the inoperative condition of the lamp bears against the strip-shapedpart 14. Said strip-shaped part 14 axially overlaps the second mainelectrode over approximately 2 mm measured from the end of theelectrode.

It has been found that the ignition voltage pulse of some twenty lampswhich are identical to the lamp described is on an average 2100 volts.The range in the value of the required ignition voltage pulse isapproximately 200 volts. In comparable lamps having an externalauxiliary means according to the prior art the required ignition voltagepulse varies from 2100 volts to 3500 volts.

What is claimed is:
 1. A high-pressure metal vapor discharge lamp havinga discharge vessel comprising a radiation-transmitting wall and firstand second main electrodes between which in the operative condition ofthe lamp the discharge takes place, said main electrodes each having anend directed towards the discharge, the discharge vessel furthermorecomprising an external ignition auxiliary means which in the inoperativelamp condition the lamp is connected electrically to said first mainelectrode and extends from the end of said first main electrode oraxially overlaps no more than 3 mm thereof and extends axially to theend of said second main electrode or to axially overlaps no more than 3mm thereof, and is also present at a distance of at most 1 mm from thewall of the discharge vessel, said external ignition auxiliary meanshaving the portion thereof near said second main electrode tightlyengaging said wall, said external ignition auxiliary means includes astrip-shaped part, disposed proximate to said second main electrode,which in the inoperative condition of said lamp said strip-shaped partof the external ignition auxiliary means tightly engages said wall ofsaid discharge vessel over at least half the circumference thereof, saidexternal ignition auxiliary means comprising a part which is movablewith respect to said discharge vessel and which in the inoperativecondition of the lamp extends along the wall of said discharge vesselbetween the ends of said main electrodes and in the operative conditionof said lamp is remote from said wall of said discharge vessel.
 2. Ahigh-pressure metal vapor discharge lamp having a discharge vesselcomprising a radiation-transmitting wall and first and second mainelectrodes between which in the operative condition of the lamp thedischarge takes place, said main electrodes each having an end directedtowards the discharge, the discharge vessel furthermore comprising anexternal ignition auxiliary means which in the inoperative lampcondition the lamp is connected electrically to said first mainelectrode and extends from the end of said first main electrode oraxially overlaps no more than 3 mm thereof and extends axially to theend of said second main electrode or to axially overlaps no more than 3mm thereof, and is also present at a distance of at most 1 mm from thewall of the discharge vessel, said external ignition auxiliary meanshaving the portion thereof near said second main electrode tightlyengaging said wall, said external ignition auxiliary means includes astrip-shaped part, disposed proximate to said second main electrode,which in the inoperative condition of said lamp said strip-shaped partof the external ignition auxiliary means tightly engages said wall ofsaid discharge vessel over at least half the circumference thereof, saidexternal ignition auxiliary means in the inoperative condition of thelamp tightly engaging the wall of the discharge vessel over a length ofat least half the distance between the ends of said main electrodesmeasured from said second main electrode, said external ignitionauxiliary means comprises a part which is movable with respect to saiddischarge vessel and which in the inoperative condition of the lampextends along the wall of said discharge vessel between the ends of saidmain electrodes and in the operative condition of said lamp is remotefrom said wall of said discharge vessel.
 3. A lamp as claimed in claim2, characterized in that the strip-shaped part is rigidly connected tothe discharge vessel and that in the inoperative condition of the lampthe electric connection between the strip-shaped part and the movablepart of the external ignition auxiliary means has an ohmic resistance ofat most 100 Ohms.
 4. A lamp as claimed in claim 1, characterized in thatthe strip-shaped part is rigidly connected to the discharge vessel andthat in the inoperative condition of the lamp the electric connectionbetween the strip-shaped part and the movable part of the externalignition auxiliary means has an ohmic resistance of at most 100 Ohms. 5.A lamp as claimed in claim 2, characterized in that the strip-shapedpart is rigidly connected to the discharge vessel and that in theinoperative condition of the lamp the electric connection between thestrip-shaped part and the movable part of the external ignitionauxiliary means has an ohmic resistance of at most 100 Ohms.
 6. Ahigh-pressure metal vapor discharge lamp having a discharge vesselcomprising a radiation-transmitting wall and first and second mainelectrodes between which in the operative condition of the lamp thedischarge takes place, said main electrodes each having an end directedtowards the discharge, the discharge vessel furthermore comprising anexternal ignition auxiliary means which in the inoperative lampcondition the lamp is connected electrically to said first mainelectrode and extends from the end of said first main electrode oraxially overlaps no more than 3 mm thereof and extends axially to theend of said second main electrode or to axially overlaps no more than 3mm thereof, and is also present at a distance of at most 1 mm from thewall of the discharge vessel, said external ignition auxiliary meanshaving the portion thereof near said second main electrode tightlyengaging said wall, said external ignition auxiliary means includes astrip-shaped part, disposed proximate to said second main electrode,which in the inoperative condition of said lamp said strip-shaped partof the external ignition auxiliary means tightly engages said wall ofsaid discharge vessel over at least half the circumference thereof,wherein said external ignition auxiliary means comprises a part which ismovable with respect to said discharge vessel and which in theinoperative condition of the lamp extends along and at least partly isin contact with the wall of said discharge vessel between the ends ofsaid electrodes and in the operative condition of said lamp is remotefrom said wall of said discharge vessel.